The 85-ton Skylab, whose slow descent from orbit threatened to bring it crashing to earth in the fall of 1979, was repositioned in space yesterday to extend its lifetime by at least six months and perhaps as long as a year.

"Everything went the way we'd planned it," said flight director William Peters at Houston's Johnson Space Center, where the maneuvers to repostion Skylab were sent by remote control to the orbiting space station.

The last maneuver was carried out at 4:28 a.m. (EDT) yesterday and involved tipping the space station in a way that it began to fly with its small end forward and parallel to the curvature of the earth. Engineers call this a "minimum drag attitude," meaning it produces as little fricion as possible with the few air particles the space station encounters at an altitude of 242 miles.

Had the 118-foot-long Skylab not been repositioned, its descent would have speeded up to a point where it could have fallen 80 miles in the next 14 months. If that happened, the space station could begin to break up in the upper reaches of the earth's atmaosphere as soon as October 1979.

Computer runs suggest that if Skylab breaks up, it could leave a trail of debris that might fall to earth along a path 3,000 miles long and 100 miles wide. Most of Skylab would burn up from the heat of friction with the atmosphere but as many as 400 pieces weighing as much as 300 pounds apiece might survive and strike the earth at speeds of almost 200 miles an hour.

The manuever that put Skylab in its "minimum drag" position yesterday was the activation of two giant gyroscopes at either end o f the space station. Flight directors ordered the gyros to turn at 9,000 revolutions a minute, more than enough to tilt the space station the way they wanted it.

Just before noon yesterday, seven hours after maneuver, flight directors at the Mission Control Center in Houston rechecked Skylab's position and found it "stable."

The space agency wants to maintain Skylab in its current position for at least 18 months so that astronauts aboard the returnable space shuttle can rendezvous with the unattended Skylab and fix a small rocket engine to its docking port.

Once shuttle astronauts do that, commands can be sent to reposition Skylab again and then fire the engine to lift it into a higher orbit where it can stay unaftended for at least 10 additional years. Even now, space engineers are conducting studies to see how astrounauts can revisit Skylab and use it 10 and 20 years hence.

Yesterday's maneuver was critical, since the earliest any astronaut crew can reach Skylab is October 1979. The shuttle is scheduled to fly next June, but the engine the astronauts will use to raise Skylab's orbit will not be tested and readied for flight until October.

The size of a four-bedroom house, Skylab is by far the largest satellite circling the earth.It was visited by three crews of astronauts, the first in 1973. The last crew left the space station almost four years ago after a stay of 85 days.